1. Field of the Invention
The present invention relates to a micro electromechanical systems (MEMS) oscillator in which a MEMS resonator made of a semiconductor material is used as a resonator.
2. Description of the Related Art
An oscillator is extensively used for generating a reference frequency used in a clock, a personal computer, or the like. A quartz resonator is generally used as a resonator in an oscillator which requires a highly accurate frequency, because of a high Q value of the oscillation frequency and temperature stability.
In recent years, depending on the application, a MEMS resonator made of a semiconductor material such as a silicon has been used instead of the quartz resonator because the MEMS resonator can be easily miniaturized and can be formed at low cost.
However, because the resonant frequency of the MEMS resonator made of a semiconductor material depends on the temperature more compared with the quartz resonator, in order to materialize a temperature-stable resonant frequency with a MEMS resonator, temperature compensation of the resonant frequency is definitely required.
For example, it is known that the resonant frequency of a MEMS resonator made of a p-type silicon decreases as the temperature rises.
By forming a dielectric film for temperature compensation, for example, a silicon oxide film, on a surface of the above-mentioned MEMS resonator, the temperature dependence of the resonant frequency is reduced, and a temperature-stable frequency can be oscillated (see, for example, JP 2007-518351 A).
More specifically, by forming on the MEMS resonator made of a silicon material a dielectric film which has the temperature characteristics of the resonant frequency, which is opposite to those of the silicon material, the temperature characteristics of the resonant frequency are improved.
However, even in a case where the above-mentioned dielectric film for temperature compensation is used, when the temperature is in the range of −40° C. to +80° C., the resonant frequency fluctuates from a reference value of the resonant frequency by about 200 ppm.
When the temperature is in the range of −40° C. to +80° C., fluctuations of the resonant frequency of an oscillator for generating a clock signal having a reference frequency, which is used in a clock, a personal computer, or the like, is required to be within a range of about 20 ppm from a reference value of the resonant frequency.
Therefore, as illustrated in FIG. 6, there is used a configuration in which fluctuations of the frequency due to temperature change is compensated for by digital signal processing of a frequency control voltage.
More specifically, as a basic configuration, a phase detector 101 determines a phase difference between a reference clock generated by a MEMS resonator 100 and an output clock output by the oscillator, a control voltage generating circuit 102 generates a control voltage for controlling the output clock from the phase difference, and a voltage control oscillator 103 uses the configuration for generating the control voltage to compensate for fluctuations of the frequency due to a temperature change by digital signal processing of the frequency control voltage.
Here, when the output clock generated by the voltage control oscillator 103 is fed back to the phase detector 101, a phase control circuit 104 controls the phase so that the frequency of the MEMS resonator 100, which is changed due to the temperature change, is corrected and outputs the corrected phase to the phase detector 101.
A storage unit 105 stores numeric values for phase control, which correspond to the temperatures, and a numeric value corresponding to the ambient temperature input from a temperature sensor 106 is output to the phase control circuit 104.
With the above-mentioned configuration, there can be formed a MEMS oscillator which is capable of making fluctuations of the resonant frequency within a range of about 20 ppm from a reference value when the temperature is in the range of −40° C. to +80° C.
When the above-mentioned digital processing is carried out, although temperature compensation of the oscillation frequency can be made with high accuracy. However, the phase control circuit carries out floating-point operations and the like, and it is necessary to provide the storage unit 105, causing problems that the circuit scale for the digital processing becomes large, the manufacturing cost is increased, and the oscillation circuit cannot be miniaturized.
Further, the above-mentioned related art has a weak point that, because a phase locked loop (PLL) is used, fluctuations with jitter is caused within a controlled range, and noise is generated.